Why Do Toilets Flush Counterclockwise In the Northern Hemisphere and Clockwise In the Southern Hemisphere?

July 9, 2015 by Karen Hill | Filed Under: Science

It’s just another one of those urban legends, probably started by an overenthusiastic physics teacher. But it’s based upon a grain of truth.

Moving fluids such as air and water are slightly affected by Earth’s rotation. The phenomenon is called the Coriolis effect, after the French mathematician Gustave Gaspard Coriolis (1792–1843), who first realized that a moving fluid on the surface of a rotating sphere (Earth, for example) would be deflected somewhat from its path.

And by the way, it’s the Coriolis effect, not the Coriolis force, as so many books and even some encyclopedias refer to it. A force is something in Nature that can move things, such as the gravitational force or a magnetic force. But the Coriolis effect doesn’t move anything; it is purely a result of two existing motions, the motion of air or water, as modified by the motion of Planet Earth.

The Coriolis effect is so weak, however, that it shows up only in huge masses of liquids and gases such as Earth’s oceans and atmosphere, where it affects winds and currents quite significantly.

But even if it were much stronger, the Coriolis effect wouldn’t show up in a toilet bowl anyway, because the water swirls around for a very different reason: water jets beneath the rim. The toilet designers shoot the water in on a tangent, so as to start it swirling. Of the two toilets in my house, one shoots the water clockwise, while the other shoots it counterclockwise. And they’re in the same hemisphere. (It’s a small house.)

On the other hand, there are no jets in a sink or bathtub, so when water goes down the drain the direction of its swirl is up for grabs. Draining water must eventually make a whirlpool that turns in one direction or the other, because as its outer portions move inward toward the drain opening, they can’t all rush straight to its center at the same time. A whirlpool is the water’s way of lining up and taking turns, while still leaving a hole in the middle for the air to come up out of the pipes. If the air didn’t have any space for rising to the surface, it would block the water from going down. But is there any hemispherical preference, no matter how small, for the direction of the swirl in a sink or bathtub?

Fill your bathroom sink or bathtub and let the water quiet down for about a week so that there are no currents or temperature differences that could possibly favor one direction over another. Now open the drain without disturbing the water in the slightest. (Good luck.) The water will begin to drain and will eventually form a whirlpool. Repeat this experiment a thousand times and record the number of times it goes clockwise and counterclockwise.

You don’t have the time or patience to do this? Good. Forget it. Your sink and bathtub are doomed to failure anyway, because the drain isn’t in the center and the water currents wouldn’t be symmetrical. Whirlpools are supposed to be circular.

Scientists who apparently had little else to do have performed this experiment with the biggest, most carefully constructed, temperature-controlled, vibration-free, automatic-central-drain-opening “bathtub” you can imagine, and have been unable to detect any consistent preference for one direction or the other. In other words, it wasn’t the Coriolis effect that determined the direction, but various other uncontrollable factors. That’s hardly surprising, though, because we can calculate the magnitude of the Coriolis effect to be expected. In a normal-sized bathtub it would be so weak that at most it could push the water around to produce about 1 revolution per day, nowhere enough to overcome the effects of inadvertently caused currents.

Here’s the nitty-gritty on how the Coriolis effect works. Picture Earth as a globe, with North America facing you. Now replace North America with a giant toilet bowl. Its drain opening will be centered somewhere in South Dakota. (No offense, Dakotans.) And let’s say that it has no water jets, so that its flushing direction can be determined entirely by Monsieur Coriolis.

The globe, toilet and all, is rotating from your left to your right, from west to east; that’s the way Earth turns. But Earth’s surface is moving faster at the equator than it is farther north, just as a horsie at the rim of a merry-go-round is going faster than one near the center. That’s because a point on the equator has much farther to travel during each rotation than a point near the North Pole does.

Thus, when you drive your car northward from anywhere in the northern hemisphere, the farther north you go, the more slowly the surface of Earth is carrying you eastward. You don’t notice this, of course, because you and your car are firmly attached to the surface of Earth and are moving along with it. Air and water, however, are different; they’re only loosely attached to Earth’s surface, and are free to slop around somewhat. That’s why the Coriolis effect can affect only air and water.

Now suppose that you are in the North American toilet bowl, floating in a rowboat somewhere south of the drain opening, say, in Texas. As you start rowing northward toward the drain (away from the equator), Earth under you is carrying you eastward more and more slowly. But your Texan inertia keeps you moving eastward at the faster Texas speed; you are outrunning Earth’s surface and getting slightly ahead of it. Net effect? Relative to Earth’s surface, you have edged eastward. You have been forced into veering slightly to your right, from northbound to slightly eastbound.

Similarly (prove it to yourself), a boat floating southward from Canada would also be deflected to its right: slightly westward. So no matter which direction the water (and your boat) starts out in on its way to the drain, if it’s in the northern hemisphere it will always be coaxed into veering to the right. And right turns go clockwise. (But don’t go away before visiting the Nitpicker’s Corner.)

I’ll spare you several more paragraphs of toilet mechanics, but let me just say that in the southern hemisphere every-thing works the opposite way. Large bodies of moving air and water receive a leftward twist, and therefore tend to swirl counterclockwise. But remember: The body of water has to be huge before you can see much effect. Oceans, yes; toilets and bathtubs, no.

Okay, so tornadoes and hurricanes really rotate counterclockwise in the northern hemisphere and clockwise in the southern hemisphere, exactly the opposite of what I just led you to believe. Just hold your horses and everything will turn out right. Or left. Whatever. Lemme ’splain it to ya.

And let’s stay in the northern hemisphere, okay? Hurricanes form in areas of low air pressure. That means that the air there is distinguishably less dense, less heavy than the air surrounding it; it’s sort of like a hole in the air. Now if, because of the Coriolis effect, all the heavier air surrounding the “hole” is glancing off it to the right, that would make the “hole” itself rotate to the left. Thus, the resulting low-pressure hurricane spins counterclockwise.

No? Well, how about this? The low-pressure zone is a roulette wheel and you are the higher-pressure air. While thrusting your hand to the right, you brush it against the wheel’s edge. Won’t that make the wheel spin to the left?

Or this: You’re pushing some kids around on one of those little playground merry-go-rounds, carousels, roundabouts, whirligigs or whatever they’re called. You push it to the right and the kids spin to the left. Right?

Or, oh, hell. Just look at the diagram.

And what about the southern hemisphere? Just interchange all the “lefts” and “rights” in the last four paragraphs and all the “clockwises” will run the other way.

BONUS: Here is your reward for reading all of the foregoing without your head spinning either clockwise or counterclockwise: I’m going to tell you why all our clocks run clockwise.

It’s because the first mechanical clocks were invented in the northern hemisphere. Not obvious? Consider this.

To an observer in the northern hemisphere, the sun is always somewhere in the southern sky. Looking southward toward the sun, a northern hemisphere observer sees it moving across the sky from east to west, which to him is from left to right. The hour hands on early clocks, and at first there were only hour hands, were intended to mimic this left-to-right movement of the sun.

Hence, they were made to move across the top of the dial in the direction that we now call “clockwise.” When the refinement of minute hands came along toward the end of the sixteenth century, they, of course, were made to go in the same direction. Can you imagine a clock with the hour hand going one way and the minute hand going the other?

If mechanical clocks had been invented in Australia, they’d all be running counterclockwise.